1. Field
Exemplary embodiments relate to optical devices including a three coupled quantum well structure. More particularly, exemplary embodiments relate to an optical device including a three coupled quantum well structure, which can achieve both low driving voltage and high optical absorption strength.
2. Description of the Related Art
In addition to taking images, three-dimensional (3D) cameras in the related art also measure distances between a plurality of points on an object and the cameras. In the related art, various algorithms for measuring a distance between an object and a 3D camera have been utilized. A time-of-flight (TOF) method is one of the most commonly used techniques. In the related art, the TOF method is used to measure the time of flight taken for light, emitted by an illumination unit to be irradiated onto an object, reflected from the object, and then received by a light receiving unit. In the related art, the time of flight of the emitted light may be obtained by measuring a phase delay between the emitted light and the received light. In the related art, a high-speed optical modulator is used for measurement of the phase delay.
In order to acquire 3D images with high distance precision, an optical modulator having excellent electro-optical response characteristics is required. To accomplish this, a gallium arsenide (GaAs)-based semiconductor optical modulator of the related art has been used. The GaAs-based semiconductor optical modulator has a P-I-N diode structure in which a multiple quantum well (MQW) structure is disposed between P- and N-electrodes. In this structure of the related art, upon application of a reverse bias voltage to both ends of the P-I-N diode, the MQW structure generates excitons in a certain wavelength region to absorb light. Since an absorption spectrum of the MQW structure tends to move toward a longer wavelength as the reverse bias voltage increases, the degree of absorption at a certain wavelength may vary depending on a change in reverse bias voltages.
Based on the above principle, it is possible to modulate the intensity of incident light of a certain wavelength by adjusting a reverse bias voltage applied to an optical modulator. The extent to which the absorption spectrum moves toward a longer wavelength may be represented as transition energy. In the related art, the transition energy is proportional to the fourth power of a thickness of one quantum well layer and the square of an applied voltage. Thus, as the thickness of one quantum well layer and the applied voltage increase, the absorption spectrum may be displaced toward a longer wavelength. If the transition energy is large, there is a large difference between the degree of absorption when a voltage is applied to an optical modulator and when no voltage is applied thereto. Thus, a high contrast ratio is obtained.
Since electro-optical properties of a GaAs-based semiconductor optical modulator in the related art may vary depending on a temperature, it is desirable to minimize heat emission by lowering a driving voltage of the optical modulator. Because a transition energy is proportional to the fourth power of a thickness of a quantum well layer and the square of an applied voltage as described above, the thickness of the quantum well layer may be increased in order to reduce the driving voltage. However, an increase in the thickness of quantum well layer may degrade an absorption strength of a MQW structure. In general, the absorption strength is inversely proportional to the thickness of the quantum well layer, but is proportional to the square of the overlap integral of wave functions of holes and electrons in the quantum well. As the thickness of the quantum well layer increases, the overlap integral of the hole and electron wave functions decreases. Thus, the number of excitons generated from electron-hole pairs may be decreased, degrading absorption strength. Therefore, when the thickness of the quantum well layer is increased in order to lower the driving voltage, the absorption strength may be reduced. Thus, the performance of the optical modulator may be degraded.
In other words, in the related art, there is a trade-off between the condition of transition energy for low voltage driving and the condition of a high absorption strength. Thus, the driving voltage of the optical modulator in the related art and the thickness of the quantum well layer may be determined by optimizing the trade-off conditions. Increasing the required performance in the optical modulator of the related art necessitates a higher driving voltage.